Transaction processors (also known as TP monitors) provide a number of middle-tier application services, including:
Transaction processing is possibly the oldest form of middleware, going all the way back to IBM's CICS. In recent years, Microsoft launched MTS (Microsoft Transaction Server), a simple transaction processor that sits at the heart of NT-based transaction systems.
Transaction Processors Vendor Tools
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Transaction processors considerations

There are an incredible number of legacy applications developed on top of transaction processors, especially CICS. In large-scale e-Commerce implementations that require legacy integration, architects will often need to integrate the e-Commerce solution with CICS or other older transaction processor-based applications.
Though transaction processors offer tried and true technology, many architects believe that their usefulness has been bypassed for new application development. For example, many of the modern publish and subscribe engines:

In new development projects it is often advisable to consider alternatives to transaction processors when implementing the architectural values of reliability, manageability, scalability, and most definitely interoperability.

Even though the mainframe, AS/400, and Unix communities developed on transaction processors for decades, the Microsoft developer community has just discovered the power of transaction processing through the release of MTS.
MTS remains a backbone architecture for Microsoft server-side processing, and much custom development in the Microsoft community has focused on MTS. Nonetheless, functionally speaking, MTS qualifies as a relatively light transaction processor, and does not operate elegantly in a cross-platform environment. Do not be afraid of suggesting other Microsoft-based alternatives for transaction engines to the Microsoft-centric e-Commerce buyer, especially when dealing with mixed platforms.

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Transaction Processor - Quiz